by Jolie Lee, USA TODAY Network

by Jolie Lee, USA TODAY Network

A 1960 letter from civil rights leader Martin Luther King Jr. to Rosa Parks went on sale Tuesday for $125,000.

"It's the only letter I'm aware of from King to Parks," said Gary Zimet, curator at Moments in Time, which is selling the letter. The website deals in historical letters and documents, as well as signed photographs and autographs.

Dated April 12, 1960, King writes that he is responding to a letter he received from Parks on March 14. Explaining his delay in responding, he writes, "An extremely crowded schedule has stood in my way."

"In the midst of constant harassment and intimidation because of my involvement in the civil right struggle I often find myself asking, 'Is it worth it?' " King writes. "But then a friend of good will comes along with kind and encouraging words that gives me renewed vigor and courage to carry on. Your letter serves such a purpose."

In 1955, Parks was arrested for refusing to give up her seat on a city bus to a white passenger in Montgomery, Ala.

The arrest sparked the 13-month Montgomery bus boycott, a protest that culminated in a Supreme Court ruling that declared segregation on public buses unconstitutional. From the protest emerged a young civil rights leader: then-26-year-old King.

Zimet did not provide details of how he received the letter but said, "It emanates from the Rosa Parks family. Beyond that, I cannot be more specific."